This disclosure relates generally to heat exchangers and, more particularly, to a microchannel heat exchanger for use in heat pump applications.
One type of refrigerant system is a heat pump. A heat pump can be utilized to heat air being delivered into an environment to be conditioned, or to cool and typically dehumidify the air delivered into the indoor environment. In a basic heat pump, a compressor compresses a refrigerant and delivers it downstream through a refrigerant flow reversing device, typically a four-way reversing valve. The refrigerant flow reversing device initially routes the refrigerant to an outdoor heat exchanger, if the heat pump is operating in a cooling mode, or to an indoor heat exchanger, if the heat pump is operating in a heating mode. From the outdoor heat exchanger, the refrigerant passes through an expansion device, and then to the indoor heat exchanger, in the cooling mode of operation. In the heating mode of operation, the refrigerant passes from the indoor heat exchanger to the expansion device and then to the outdoor heat exchanger. In either case, the refrigerant is routed through the refrigerant flow reversing device back into the compressor. The heat pump may utilize a single bi-directional expansion device or two separate expansion devices.
In recent years, much interest and design effort has been focused on the efficient operation of the heat exchangers (indoor and outdoor) in heat pumps. High effectiveness of the refrigerant system heat exchangers directly translates into the augmented system efficiency and reduced life-time cost. One relatively recent advancement in heat exchanger technology is the development and application of parallel flow, microchannel or minichannel heat exchangers, as the indoor and outdoor heat exchangers.
These parallel flow heat exchangers are provided with a plurality of parallel heat transfer tubes, typically of a non-round shape, among which refrigerant is distributed and flown in a parallel manner. The heat exchanger tubes typically incorporate multiple channels and are oriented substantially perpendicular to a refrigerant flow direction in the inlet and outlet manifolds that are in communication with the heat transfer tubes. Heat transfer enhancing fins are typically disposed between and rigidly attached to the heat exchanger tubes. The primary reasons for the employment of the parallel flow heat exchangers, which usually have aluminum furnace-brazed construction, are related to their superior performance, high degree of compactness, structural rigidity, and enhanced resistance to corrosion.
The growing use of low global warming potential refrigerants introduces another challenge related to refrigerant charge reduction. Current legislation limits the amount of charge of refrigerant systems, and heat exchangers in particular, containing most low global warming potential refrigerants (classified as A2L substances). Microchannel heat exchangers have a small internal volume and therefore store less refrigerant charge than conventional round tube plate fin heat exchangers. In addition, the refrigerant charge contained in the manifolds of the microchannel heat exchanger is a significant portion, about a half, of the total heat exchanger charge. As a result, the refrigerant charge reduction potential of the heat exchanger is limited.